Gas scrubbing systems for removing unwanted constituents from gas streams are utilized in many different industries such as the chemical and paper industries, water and waste water treatment plants, and other applications where unwanted constituents are present in a process gas. The scrubbers are designed to treat the gas before it is released into the atmosphere. In this manner, unwanted constituents may be removed, or neutralized, before the gas is released.
In general, such scrubbing systems are utilized under conditions in which predictable volumes of gas are passed therethrough for treatment. Such systems, however, are not at all suitable for removing unwanted noxious fluids when they are released in large concentrations, under explosive conditions, such as during the accidental explosive release of a noxious fluid under pressure. In such cases, conventional scrubber systems are unable to reduce the level of unwanted constituents to tolerable levels within a sufficiently short period of time.
It is well known that compressed noxious fluids such as chlorine, ammonia, sulfur dioxide and hydrogen chloride gases are used extensively in a variety of industrial applications. Chlorine, for example, is widely used in chemical industries and in water treatment plants. These gases, while having substantial utility in industrial processes, present serious health problems, sometimes life threatening, if they are released in an uncontrolled manner. Thus, it is very important in the utilization of such gases to consider requirements for prevention, control and mitigation of dangerous conditions relating to their use.
In recognition of safety considerations, cylinders containing a noxious gas are often stored in a ventilated enclosure such as a gas cabinet, or storage room. Such an enclosure is generally maintained normally at a negative pressure in relation to the surrounding area to reduce the likelihood of leakage of a corrosive gas from the storage area. It is recognized that when a leak from a pressurized cylinder occurs, room pressure rises because of the added vapor therein. Thus, to maintain a negative pressure in the room or cabinet, a ventilation exhaust rate must be established that is higher than the gas vapor generation rate. In some situations, when the leak rate is not substantial, this can be accomplished by exhausting the air through a conventional scrubber, before it is vented to the atmosphere. However, conventional scrubbers are unable to handle large concentrations of noxious gas, released in a very rapid manner.
Thus, it is recognized that under emergency conditions, when very large volumes of noxious fluids are explosively released inadvertently , a suitable emergency system must be capable of reducing the discharge concentration of the escaping fluid to an acceptable level within a very short period of time. This level, representing the concentration of airborne contaminants, is normally expressed in parts per million (ppm). In the case of chlorine, for example, the maximum acceptable chlorine concentration, at the exit of an emergency chlorine treatment system, is generally regarded to be about 15 ppm.
Conventional scrubber systems are capable of reducing chlorine concentrations in an enclosure to the 15 ppm level when a small leak is involved. However, such systems are not capable of processing an enclosure having very large concentrations of chlorine, when the chlorine is released in an explosive manner from large storage cylinders. In this regard, when establishing emergency system performance criteria, the entire content of a tank or cylinder must be considered.
In many commercial applications, a cylinder containing about 2,350 pounds of liquified chlorine is used. In general, the cylinders are constructed of steel and are equipped with one or more pressure relief devices. It is known that, in spite of careful design of such containers, leaks occur because of human error or because of failure of some component in the storage system. However, leaks occurring through valve packing, threads, gaskets and valve seats generally do not result in catastrophic failures and may not require the use of an emergency scrubbing system. However, when chlorine release is the result of valve failure, a blown fusible plug or puncture of a cylinder wall, large volumes of liquid and gaseous chlorine are released in a very short time. The results can be life threatening for those in the vicinity of the spill. In such cases, the chlorine contaminated enclosure and surrounding areas must be treated immediately on an emergency basis.
To date, attempts have been made to adapt conventional techniques to an emergency release situation. In this regard, a dilution/dispersion technique has been considered wherein contaminated air from a storage room is vented through an exit stack to the atmosphere, without any scrubbing or chlorine neutralization. The exit gas stream entrains ambient air, thereby diluting the chlorine concentration.
Such a method may protect local plant personnel in the immediate vicinity of the storage room. However, it is unacceptable for many applications, since it presents a dangerous condition to the health of people living downwind of the plant. As a result, such a technique is incompatible with public safety and is often prohibited by ordinance.
A recycling chlorine scrubber system is another proposed method for attempting to solve the problem. In this system, air is withdrawn from the storage room at a rate higher than the chlorine vapor release rate, passed through a scrubber for chlorine removal and then recycled back to the storage room.
However, the technique presents several serious drawbacks due to the highly corrosive characteristics of the fluid to be treated. By recycling air from the scrubber, severe corrosion to equipment in the storage room can occur. In addition, it is difficult to maintain negative pressure in the room and, as a result, corrosive gas leakage from the storage room may occur, thereby threatening the health of personnel in the vicinity of the room. Thus, the recycling type of scrubber is not suitable for the emergency condition for many applications.
In contrast to recycling systems, once-through emergency scrubbing systems have also been considered. In such systems, storage room air is exhausted through a scrubber for removal of noxious gas, such as chlorine, before it is vented to the atmosphere. In such cases, the exhaust rate must be greater than the highest chlorine vapor generation rate to assure that a negative pressure is maintained in the storage room and to prevent unwanted chlorine vapor from escaping to the atmosphere.
This technique is relatively simple in operation, but it requires a very efficient scrubber system, having a removal efficiency greater than 99.998%, in order to maintain chlorine levels at 15 ppm at the scrubber exit. Such scrubber efficiencies have been heretofore unknown in conventional scrubbing systems of any kind.
In view of the foregoing, while a once-through system has attractive features, the high efficiency requirement presents a substantial limitation. In view of this fact, it would be highly desirable to have an emergency scrubber having the advantages of a once-through system, with very high scrubber efficiencies capable of safe and economic operation.
In addition to the above described techniques, other conventional scrubbers utilize a packed tower having an induced draft fan to exhaust the storage room air. The packed tower is often a counter flow vertical tower having random packing which is irrigated at the top with a caustic solution. Such a system usually requires an unduly large size system for some applications. Also, because of the height of such conventional packed towers, a time delay of a minute or two may be required for the caustic solution to wet the packing completely. Of course, until the packing is wetted, it does not contribute to the scrubbing of the noxious fluid. Therefore, during this initial wetting period when scrubbing requirements are at the highest, scrubber efficiency is very low. Thus, the packed tower system is not acceptable for many applications.
Another conventional approach is to use an ejector Venturi to evacuate the storage room. Typically, the Venturi is mounted over a tank containing caustic reagents and discharges into the tank. In the event of a leak of noxious fluid, such as chlorine leak, a high pressure pump injects a high flow of caustic solution into the Venturi throat, thereby creating a suction and causing the air flow out of the storage room. The caustic solution atomizes into drops providing surfaces for chlorine absorption. Most injected caustic solution is separated from the gas stream by impacting on the liquid surface in the tank and by gas flow direction changes.
A severe disadvantage of such a system is low chlorine removal efficiency. The short contact time in the ejector and large liquid drops formed therein results in a chlorine removal efficiency of a conventional ejector Venturi of between about 70-80%. Such a level of performance is unacceptable in emergency conditions. In an attempt to overcome the low removal capability, ejector Venturis technique are sometimes utilized in combination with a recycling system. This approach, however introduces the severe drawbacks of recycling systems.
In some conventional systems, a packed tower is added downstream of an ejector Venturi in an attempt to improve the scrubbing operation. Although this design may eliminate some of the shortcomings of a recycling system, the packed tower in such applications must be undesirably large in size. For instance, since chlorine vapor concentration can be as high as about 800,000 ppm during the first minute of a catastrophic failure, if the ejector removal is 80%, the chlorine concentration at the packed tower inlet is about 160,000 ppm.
In order to reduce the vapor concentration to the desired 15 ppm at the outlet, the required removal efficiency for the packed tower would require a tower height exceeding 11 feet. Such a system is not only expensive to manufacture, it occupies unreasonably large areas for some applications.
Another important factor to be considered is the nature of the neutralization reaction which is usually exothermic, thereby producing substantial amounts of heat in a very short time period when chlorine, for example, reacts with a caustic substance. In such cases, "hot spots" can develop, as gas flows through system ducts, because of a concentration of the gas near the center of the duct. This causes an uneven flow distribution within the duct, and an inefficient scrubbing operation, because the caustic reagent is incapable of reacting uniformly with the chlorine gas. An ineffective and incomplete reaction is thus realized.
Another critical factor to be considered is the fact that the extremely high concentrations of the rapidly escaping chlorine gas flowing through the scrubbing system could be many times greater than a lethal dosage. Thus, should the gas escape from the scrubbing system, people in the vicinity of the scrubbing system would be in grave danger of losing their lives. In the event of an unwanted leak or other similar type of malfunction, in the emergency scrubbing system, personnel within the plant would be exposed to life threatening conditions.
Similarly, should the emergency scrubbing system fail or otherwise malfunction, such as by the bursting or leaking of the treatment conduits, the system would be unable to function to such an expected high degree of efficiency and effectiveness. Also, a bursting or leaking treatment conduit can cause dangerous treating fluids, such a caustics, to be sprayed in the direction of personnel within the immediate vicinity, thereby causing another unwanted threat to human life.
A still further consideration in the design of an emergency scrubbing system, is the provision of an expensive emergency electrical power generation subsystem. Such subsystems are very expensive, due to their fail-safe design. Thus, it would be highly desirable to have a new and improved emergency scrubbing system, which can be operated electrically from smaller, less expensive emergency electrical power generation subsystems.
In view of the foregoing, it would be highly desirable to have an efficient and effective emergency fluid scrubbing system which could combine extremely high efficiency of operation with a relatively compact low height size to render safe and harmless massive concentrations of rapidly expanding lethal fluids, such as chlorine gas. Such an emergency scrubbing system would protect people in the vicinity of the inadvertent and unwanted massive release of the explosive fluid. In the case of chlorine gas, inhaling a few breath of a high concentration, such as about 1,000 ppm, of the gas could in at least some situations, cause immediate death, or at least serious bodily injury. It is possible that an explosive release of chlorine gas could result in many times higher concentrations, such as about 400,000 ppm.
Thus, a new and improved emergency scrubbing system should be about 99.999 percent efficient, to reduce the concentrations to relatively safe conditions. Such efficiencies have been heretofore unknown in a relatively compact and low cost system.
Also, such a new and improved emergency scrubbing system should be able to confine within it, to a reasonable extent, both the deadly fluids under treatment flowing through it, as well as the dangerous treating fluids being delivered to the fluids under treatment. Moreover, such a new emergency scrubbing system should be able to be powered by a small lower cost electrical power subsystem. Preferably, such a system would include the advantages afforded by a system capable of performance on a once-through basis and would avoid the problem of ineffective and incomplete neutralization reactions within the system.